I tried to determine what the actual size of some rangefinding circles, bars, posts might be in several scopes a while back. Drew some simple bars with a felt pen at 16" and 18", then also stood a wooden yard stick up with a line drawn on the cardboard where the 0 would be. Found that some circles that were supposed to be 18" were that size on the outside of the circle, but my eye prefered to use the inside. The circle was 0.5" thick so the inside circle was only 17" across.

Not sure why the scope companies do not standardize on the distance between duplex reticle posts, some are 18", some are 16". Too bad there wasn't 16, 18, 20, 22, 24 inch indicators since 16 and even 18 are too smal for big whitetails and mulies for a brisket to backline comparison.
Good luck with your efforts, hope that they work out OK.

Yeah Ian, it is a shame they don't provide a good std. to work with. I'm trying to get my plex measurements as accurate as possible for ranging, zeroing, windage applications and changes in all 3 with magnification changes. I used to have a sign post with holes drilled in it at 1" intervals out the front window of my house @ 102 yds. that provided a good reference system to use, but we moved so that option's out. I wanted to get measurements down to .1", but i couldn't get enuf resolving power anyway for that. The websites/literature are another option, but sometimes they're not accurate-- case in point is the Burris Ballistic Plex, horizontal plex post to post "gap" is not what they advertise in my scopes. It's supposed to be 4.1" for the 3-12X LER, but i've found it to measure about 6" in 1 of mine. They always seem to be right regarding the ballistc stadia, but not always with the plex gap measurements. I've also read that sometimes the mil-dot is not correct @ the calibrated magnification, but i checked my 6-24X Burris Ballistic Mil-dot, and it's right on the nose, and seems to be reproducible. I actually have another sign post out my new window @ 119 yds. that gives me a std. that i just started to work with, but again probably only accurate to .5" maybe. Oh well, i guess i'll just have to punt for now. I recently started applying a windage reference system using the horizontal plex post as a relative measurement in tenths of a plex "unit" that's working pretty well for me, so i guess maybe i don't have to get that accurate. "Punting" seems to be working, but i certainly welcome a more accurate system.
U know, come to think of it i might just give Michael O'Donnell a call @ TK Lee. He's a pretty good guy who might be able to shed some light on the subject. Thks. guys for the response's.

SS,
This might sound hokey but here is how I check mildot spacings at 100 yards.
I found an old plastic mop-handle, it happens to be a bright blue color. I took electricians tape and wound a 3.6" section, measured 3.6" again then started another 3.6" taped section and continued this. I just stand the handle up at 100 yards and we can place the bottom post on the bottom of the first black section, the dots hit the spacings or if they are not accurate they miss them. Looks a bit like a blue and black barber pole.

A person could do this easily with other spacings, black and a contrasting color on a pole would work good.

I also made heavy black bars spaced at 3.6" on a piece of cardboard and posted it at 100 but the blue/black handle works better.

SS,
Have not had any good quality production scopes show any mildot spacing errors. Have had prototypes do that but they were early production samples that needed some improvements. Have read about some low priced scopes being significantly out with their mildot spacing, have not experienced that.

Spacing boo-boos happen, everyone should check their duplex's, bars, circles, dots etc. instead of just trusting that they are what is claimed.

Interesting how the round dots have won the mildot competition with most manufacturers, even Leupold is using them in most of their scopes now.